Laser emitters are widely used preparatory to radiation treatment of a patient for accurately positioning a target area on the patient relative to the isocenter of a radiotherapy machine. This use of the target device is an important application of the present invention, although it is not the only one. The target device can be used in other situations in which there is a requirement to align a plurality of beams from lasers or other light emitters to cause them to converge at an isocenter.
In the medical application referred to above, namely when the invention is used in conjunction with radiotherapy equipment, there are typically four light sources, e.g. laser sets. One laser set is mounted on each of two opposite walls of the room in which the equipment is located, and they are oriented to direct composite beams horizontally towards each other. These are referred to as the lateral laser sets. A third laser set is mounted on the ceiling and directs its composite beam downwardly. Ideally, the three sets of beams should intersect in space to define the isocenter. The fourth laser, called the saggital laser, is mounted on a third wall of the room at an elevated location, and directs its single beam towards the isocenter at right angles to the two lateral composite beams coming from the wall mounted laser sets.
The composite beam from each of the lateral and ceiling laser sets consists of two intersecting portions, each portion being a narrow strip. The two strips are perpendicular to each other, one strip being horizontal and the other vertical. The two beam portions thus define a cross. In the case of the ceiling laser set, the two strips are both horizontal. The single beam from the saggital laser consists of only one portion, namely a vertical strip.
The beams are visible to the operator and are designed to meet and display a cross on a common target.
The foregoing procedure is known in the art, but the prior art devices for achieving alignment of the beams to define the isocenter have suffered from inaccuracies. They have generally relied on use of a pair of right-angled wires (crosshairs) that project a shadow image of a cross on whatever target is used. A problem when depending on these crosshairs is that on some radiotherapy machines they are mounted on a removable tray that slides in and out with provision for adjusting its position in the X and Y directions (the two horizontal directions). Since the prior art laser alignment devices have depended on proper crosshair alignment in the initial set up, if the crosshairs are in fact not correctly aligned, the lasers will also lack proper alignment. Another problem is that correct alignment of the crosshairs depends on the operator's visual dexterity. In addition, the alignment checks are performed sequentially, each subsequent check requiring a new set up.
Yet another problem of the prior art alignment techniques resides in the fact that, even assuming that all the beams accurately intersect at the isocenter, there is no guarantee that they are accurately directed relative to each other, which is a requirement of a perfect set up. Specifically, the two lateral beams should together define a single straight line and the ceiling beam should be at right angles to the lateral beams.